A known technique of automatic frequency control for superheterodyne radio receivers designed for receiving binary digital signals, which has been used successfully, is to measure the frequency of a converted signal in the receiver, the converted signal being a signal generated by a mixing operation performed in the receiver which converts the frequency of a modulated RF carrier to a modulated signal at a lower, intermediate frequency (IF). In this approach, the modulated IF signal is measured for a plurality of bit periods, and an average IF frequency is determined from the frequency measurements. This is then used to modify the mixing operation by controlling a local oscillator frequency, which improves the receive sensitivity.
A different technique of automatic frequency control is known for use in zero IF quadrature receivers. This technique is most appropriate for receiving frequency modulation signals having low deviation-to-bitrate ratios, such as Minimal Shift Keying (MSK) and Gaussian Frequency Shift Keying (GFSK). In this technique, time durations between the bit transitions and clock transitions of the quadrature signals and a nominal time are used as a basis for controlling the local oscillator to improve the receive sensitivity. While this technique is appropriate for MSK and GFSK, it requires many calculations per bit period in systems using frequency modulation techniques having higher deviation-to-bit-rate ratios, thus causing the use of higher speed, more costly processors or logic to accomplish the improvement.
The techniques described above are typically enabled within the receiver only when a sufficiently strong signal is available, in order to prevent random corrections from occurring when no signal is being received. This has been previously accomplished by enabling and disabling the automatic frequency control by means of a signal strength indication generated within the receiver, or by the use of a selective call address to enable the automatic frequency control when the signal being received is intended for the radio receiver.
While this has worked satisfactorily, it can result in the automatic frequency control being disabled for long periods of time while a signal is being received which happens not to be intended for the radio receiver, even though an intended signal will be received from the same source, and can result in corrections occurring during the receipt of a signal which are of sufficient magnitude to induce errors which otherwise would not be induced.
Thus, what is needed is a simple, low cost, means of automatic frequency control well suited for portable radio communication receivers.